1. Field of the Invention
This invention relates to an information recording medium which performs optically recording and reproduction of information.
2. Related Background Art
In recent years, with the progress of information service in society, as a means which can deal with a diversity of informations with good efficiency, there have been proposed a large number of information recording carriers as well as recording and reproducing devices which can perform recording or reproduction of optical information such as optical disc, optical card, optical tape, etc. Among the above information recording carriers, there are those in which information converted to bivalue can be detected through exchange of the change in intensity of reflected light accompanied with change in surface shape such as presence of pit.
As the recording carrier according to the change in optical reflectance, there has been proposed the so called heat mode recording material which records a part of the recording layer through the change in state by irradiating the recording layer with an energy beam such as laser beam in spots. These recording materials are so called DRAW (direct read after write) which can "immediately read after writing" without requiring developing processing, etc. after writing of information, which can perform high density recording and also can perform additional writing.
In an optical recording and reproduction device of information, for track servo of writing and reading, there is the system in which grooves are formed previously at predetermined intervals on the surface of a substrate. According to this system, grooves play the role of guide for reading, and therefore the track control precision of laser beam can be improved, whereby higher speed access is rendered possible than the system of the prior art in which a substrate which has no grooves is employed.
For formation of track grooves on a substrate, when the substrate is a thermoplastic resin, there have been known in the art the method in which a stamper form is heat transferred according such method as injection molding, hot press molding, etc. at a temperature of the melting point or higher, or the method in which a stamper form is optically transferred by coating a photocurable resin composition on the substrate, bringing a stamper mold into close contact therewith and curing said photocurable resin composition by irradaition of UV-ray, etc. thereby to transfer optically the stamper form.
FIG. 3 is a schematic sectional view of an optical card which is one of information recording media of the prior art. In FIG. 3, 6 is a transparent resin substrate, 7 a track portion, 8 a recording layer, 4 an adhesive layer and 5 a card substrate. In FIG. 3, recording and reproduction are performed by way of optical writing and reading through the transparent resin substrate 6 and the track portion 7.
In the card of the prior art, in the case of the heat transfer method as described above, the track portion 7 is integrated with the transparent resin substrate 6 to be of the same material, while in the case of the optical transfer method, the track portion 7 is not of the same material as the transparent resin substrate 6, but in the adhered state. Anyway, track groove corresponding to the guide groove for light is previously formed so as to effect tracking through the phase difference of laser beam by utilizing fine unevenness. Also, the optical card can be manufactured easily by forming an optical recording layer 8 on the track portion 7 and plastering a card substrate 5 through the adhesive layer 4.
As described above, the optical card of the prior art has the constitution such that an optical recording layer is formed on the transparent resin substrate provided with a track portion comprising unevenness shape, and therefore first an unevenness shape must be formed on the transparent resin substrate.
In that case, the yield of transfer of the stamper form, the transfer time, durability and cost of the stamper form, etc. pose problems in making bulk productivity of the optical card difficult, and also increasing the cost.
Further, since the recording sensitivity of the optical recording layer depends on the film thickness, uniform film formation is required. However, it is difficult to obtain uniformity of film thickness, because an optical recording layer is to be formed on the uneven shape of the track portion. Also, there has been also involved the problem that reproducibility of the shape after formation of the optical recording layer is difficult, because the optical recording layer is formed on the uneven shape.
As the method for solving these problems, there has been known the method in which address pits, etc. are directly formed by irradiation of laser beam on the recording layer of Te as described in U.S. Pat. No. 4,428,075. However, according to this method, there has been the problem that the recording layer material scattered is attached on the region for writing information to form noise.
As the method which has solved such problem, there is disclosed in Japanese Patent Laid-open Application No. 59-171046 the method in which there are provided a dye film which is deformed by absorption of visible light on the substrate and a reflective film which records information thereon, and visible laser is irradiated to shrink the dye film, thereby forming a track guide groove. However, according to this method, two layers of the dye film and the reflective film which records information must be formed, and also productivity is poor because the track grooves are formed by scanning laser one by one.